This invention relates generally to surgical techniques and prosthetic components therefor and, in particular, to intervertebral disc replacement apparatus and methods of implanting the same.
Eighty-five percent of the population will experience low back pain at some point. Fortunately, the majority of people recover from their back pain with a combination of benign neglect, rest, exercise, medication, physical therapy, or chiropractic care. A small percent of the population will suffer chronic low back pain. The cost of treatment of patients with spinal disorders plus the patient""s lost productivity is estimated at 25 to 100 billion dollars annually.
Seven cervical (neck), 12 thoracic, and 5 lumbar (low back) vertebrae form the normal human spine. Intervertebral discs reside between adjacent vertebra with two exceptions. First, the articulation between the first two cervical vertebrae does not contain a disc. Second, a disc lies between the last lumbar vertebra and the sacrum (a portion of the pelvis).
The spine supports the body, and protects the spinal cord and nerves. The vertebrae of the spine are also supported by ligaments, tendons, and muscles which allow movement (flexion, extension, lateral bending, and rotation). Motion between vertebrae occurs through the disc and two facet joints. The disc lies in the front or anterior portion of the spine. The facet joints lie laterally on either side of the posterior portion of the spine.
The human intervertebral disc is an oval to kidney bean shaped structure of variable size depending on the location in the spine. The outer portion of the disc is known as the annulus fibrosis. The annulus is formed of 10 to 60 fibrous bands. The fibers in the bands alternate their direction of orientation by 30 degrees between each band. The orientation serves to control vertebral motion (one half of the bands tighten to check motion when the vertebra above or below the disc are turned in either direction). The annulus contains the nucleus. The nucleus pulpous serves to transmit and dampen axial loads. A high water content (70-80 percent) assists the nucleus in this function. The water content has a diurnal variation. The nucleus imbibes water while a person lies recumbent. Activity squeezes fluid from the disc. Nuclear material removed from the body and placed into water will imbibe water swelling to several times its normal size. The nucleus comprises roughly 50 percent of the entire disc. The nucleus contains cells (chondrocytes and fibrocytes) and proteoglycans (chondroitin sulfate and keratin sulfate). The cell density in the nucleus is on the order of 4,000 cells per micro liter.
Interestingly, the adult disc is the largest avascular structure in the human body. Given the lack of vascularity, the nucleus is not exposed to the body""s immune system. Most cells in the nucleus obtain their nutrition and fluid exchange through diffusion from small blood vessels in adjacent vertebra.
The disc changes with aging. As a person ages the water content of the disc falls from approximately 85 percent at birth to 70 percent in the elderly. The ratio of chondroitin sulfate to keratin sulfate decreases with age. The ratio of chondroitin 6 sulfate to chondroitin 4 sulfate increases with age. The distinction between the annulus and the nucleus decreases with age. These changes are known as disc degeneration. Generally disc degeneration is painless.
Premature or accelerated disc degeneration is known as degenerative disc disease. A large portion of patients suffering from chronic low back pain are thought to have this condition. As the disc degenerates, the nucleus and annulus functions are compromised.
The nucleus becomes thinner and less able to handle compression loads. The annulus fibers become redundant as the nucleus shrinks. The redundant annular fibers are less effective in controlling vertebral motion. The disc pathology can result in: 1) bulging of the annulus into the spinal cord or nerves; 2) narrowing of the space between the vertebra where the nerves exit; 3) tears of the annulus as abnormal loads are transmitted to the annulus and the annulus is subjected to excessive motion between vertebra; and 4) disc herniation or extrusion of the nucleus through complete annular tears.
Current surgical treatments of disc degeneration are destructive. One group of procedures removes the nucleus or a portion of the nucleus; lumbar discectomy falls in this category. A second group of procedures destroy nuclear material; Chymopapin (an enzyme) injection, laser discectomy, and thermal therapy (heat treatment to denature proteins) fall in this category. A third group, spinal fusion procedures either remove the disc or the disc""s function by connecting two or more vertebra together with bone. These destructive procedures lead to acceleration of disc degeneration. The first two groups of procedures compromise the treated disc. Fusion procedures transmit additional stress to the adjacent discs. The additional stress results in premature disc degeneration of the adjacent discs.
Prosthetic disc replacement offers many advantages. The prosthetic disc attempts to eliminate a patient""s pain while preserving the disc""s function. Current prosthetic disc implants, however, either replace the nucleus or the nucleus and the annulus. Both types of current procedures remove the degenerated disc component to allow room for the prosthetic component. Although the use of resilient materials has been proposed, the need remains for further improvements in the way in which prosthetic components are incorporated into the disc space, and in materials to ensure strength and longevity. Such improvements are necessary, since the prosthesis may be subjected to 100,000,000 compression cycles over the life of the implant.
This invention resides in artificial replacements for natural intervertebral discs in humans and animals. Broadly, the invention comprises a shaped body having a final volume sized to consume at least a portion of the intervertebral disc space, and a material associated with the shaped body enabling the body to cyclically compress and expand in a manner similar to the disc material being replaced. The body may be composed of a compressible material, such as polymeric urethane or other suitable elastomers, or may include a filling to impart an appropriate level of compressibility. In any case, the body preferably features some form of collapsed state permitting easier insertion, and a final state having superior and inferior surfaces preferably conformal to the concavities of the vertebral endplates. The superior and inferior surfaces may accordingly be convex, and may further include grooves, spikes, or other protrusions to maintain the body within the intervertebral space. The body may further be wedge-shaped to help restore or maintain lordosis, particularly if the prosthesis is introduced into the cervical or lumbar regions of the spine.
To enhance strength or longevity, the body may further include the use of fiber-reinforced materials on one or more outer surfaces or wall structures, as the case may be. Similar to commercial tire construction, such fiber-reinforced materials may be of a bias-ply, radial-ply or bias-belted construction. According to one configuration, an artificial disc according to the invention may further include an outer compressible member peripherally attached to a central xe2x80x9chub,xe2x80x9d similar, at least in concept, to the which a tire is mounted onto a wheel.
In a preferred embodiment, an artificial disc according to the invention is inflated, preferably after the body is inserted into the disc space, so that the device may assume an initial, collapsed state affording easier insertion. A gas, liquid, gel, foam or other compressible material may be used to expand the body, and the material may be introduced or otherwise provided through the use of a valve, port, syringe, or, alternatively, by way of valveless means. The body in this case is preferably a sealed unit, and may include self-sealing means in the event of a leak or rupture.
If a valve is used, it will preferably be configured so as to be accessible during implantation, enabling the surgeon to expand the device in situ. A valve may also be provided in the form of a port enabling subcutaneous post-operative inflation or re-expansion. If a gel is used as the filler material, it is preferably in the form of a hydrogel, enabling water to be imbibed and expelled to facilitate cushioning. Such a gel is preferably introduced within the body in a dehydrated state prior to implantation, with water being added to expand the material. The liquid may be added through a valve, port or hypodermic in conjunction within a sealed structure or, alternatively, at least a portion of the surface of the body, preferably the superior end or inferior surfaces, may be at least semi-porous. As a further alternative to a valveless structure, one or more reactants may be provided with the body, such that when mixed with one or more other reactants, a gas or foam is generated to expand and fill the body. As yet a further alternative, an ampule or cartridge operative to release a compressed gas or generate a gas, liquid or foam may be activated by an external source of energy such as ultrasound, heat, or other stimuli.
In terms of operative procedure, disc replacements according to the invention may be introduced through an anterior, posterior, or lateral approach using an appropriate surgical technique, including arthroscopic, laparoscopic, or microscope-assisted procedures. In the case of a non-inflatable embodiment, the prosthesis is preferably compressed to facilitate easier insertion into the disc space. The vertebrae may also be distracted, or the prosthesis may be cooled to ease implantation. More than one artificial discs according to the invention may be introduced into the same disc space, and may be arranged side-by-side laterally, or anterior to posterior. Separate flaps may be formed in the annulus fibrosis, and one or more bodies may be introduced and inflated, or allowed to expand, to at least partially distract the vertebrae to ease the insertion of additional bodies. The material of the annulus fibrosis may be reattached to maintain the disc replacement material, or synthetic bands, fabrics, or plates may be added, as required. In the event that multiple prostheses are used within a common disc space, the bodies may include interlocking shapes or structures of varying design to enhance their physical cooperation. In addition, if the replacements are arranged from anterior to posterior, devices more posterior may be smaller or of a lesser xe2x80x9cdurometer,xe2x80x9d or those placed anteriorally may be larger or less compressible (or both), again, to enhance an overall lordotic shape.